Google Search is testing a filter button for shopping related query refinement. I personally do not see this but Saad AK posted a screencast of this, showing a filter button at the top left of the search results after searching for [jeans].

When you click on the filter button, you are presented with a ton of shopping related refinements from brand, length, leg style, material, pattern, style, specialty, reviews, nearby, sales, and more.

Here are some screenshots followed by the video cast:

Then when you click on the filter button, a filter section comes up:

Forum discussion at Twitter.